more than one sf_buf for one vm_page. To accomplish this, we add
a global hash table mapping vm_pages to sf_bufs and a reference
count to each sf_buf. (This is similar to the patches for RELENG_4
at http://www.cs.princeton.edu/~yruan/debox/.)
For the uninitiated, an sf_buf is nothing more than a kernel virtual
address that is used for temporary virtual-to-physical mappings by
sendfile(2) and zero-copy sockets. As such, there is no reason for
one vm_page to have several sf_bufs mapping it. In fact, using more
than one sf_buf for a single vm_page increases the likelihood that
sendfile(2) blocks, hurting throughput.
(See http://www.cs.princeton.edu/~yruan/debox/.)
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